A light diffusing element is typically a sheet-shaped optical element including a resin matrix and fine particles dispersed in the matrix, and expresses light diffusibility on the basis of a difference in refractive index between each of the fine particles and the matrix (see, for example, Patent Literature 1). The light diffusing element has been widely utilized in, for example, a lighting cover, a screen for a projection television, or a surface-emitting device (e.g., a liquid crystal display device). In any one of the applications of the light diffusing element, such characteristics as the realization of high image quality or high lighting quality, an improvement in efficiency for light utilization, and industrially excellent production efficiency have been required. The light diffusing element needs to have sufficient light diffusibility to realize high image quality or high lighting quality. However, when the difference in refractive index between each of the fine particles and the matrix is enlarged for improving the light diffusibility, backscattering enlarges. Therefore, it is difficult for the conventional light diffusing element obtained by simply dispersing the fine particles in the matrix to realize high image quality or high lighting quality. Further, the enlargement of the backscattering in the conventional light diffusing element is incompatible with a recent worldwide trend toward the reduction of an environmental load because such enlargement reduces the efficiency for light utilization. Various technologies (e.g., a light diffusing element obtained by dispersing so-called gradient index (GRIN) fine particles in which a refractive index continuously changes from the center portion, of a fine particle toward its outside) have heretofore been proposed for improving the characteristics of the light diffusing element (see, for example, Patent Literatures 2 to 8). However, none of the technologies has provided a light diffusing element satisfying both sufficient light diffusibility and low backscattering.
As described above, realization of both sufficient light diffusibility and low backscattering is a problem that has long been unsolved in the light diffusing element, and the establishment of a technology by which a light diffusing element satisfying both the two characteristics is obtained with industrially excellent production efficiency has been strongly demanded.